Science (kinda) time!
The mist only changes things to make people see what they can understand. Children, however, believe in basically any magical thing. So, its normal for a small child to see a pegasus

No, that's not fair, he didn't know they were dating. They should've told him first. If he knew, well that's another story.
*Hides under cover* I don't like this Percy, He's mean! But I still lurrved the chapter